(a) Field of the Invention
The present invention relates to a luminescent display, and a driving method and pixel circuit thereof. More specifically, the present invention relates to an organic electroluminescent (hereinafter referred to as “EL”) display.
(b) Description of the Related Art
In general, an organic EL display is a display that emits light by electrical excitation of fluorescent organic compound and displays images by driving each of N×M organic luminescent cells with voltage or current. These organic luminescent cells have a structure that includes an anode (indium tin oxide: ITO) layer, an organic thin film, and a cathode (metal) layer. For a good electron-hole balance to enhance luminescent efficiency, the organic thin film is of a multi-layer structure that includes an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL). The multi-layer structure can also include an electron injecting layer (EIL), and a hole injecting layer (HIL).
There are two driving methods for the organic luminescent cells: one is a passive matrix driving method and the other is an active matrix driving method using TFTs or MOSFETs. In the passive matrix driving method, anode and cathode stripes are arranged perpendicular to each other to selectively drive the lines. Contrarily, in the active matrix driving method, a TFT and a capacitor are coupled to each ITO pixel electrode to sustain a voltage by the capacity of the capacitor.
FIG. 1 is a circuit diagram of a conventional pixel circuit for driving an organic EL element using TFTs. For simplicity reasons, only one of the N×M pixels is shown in FIG. 1.
As illustrated in FIG. 1, a current-driven transistor M2 is coupled to the organic EL element (OLED) to supply a current for light emission. The amount of current of the current-driven transistor M2 is controlled by the data voltage applied through a switching transistor M1. Here, a capacitor Cst for sustaining the applied voltage for a predetermined time period is coupled between the source and gate of the transistor M2. The gate of the transistor M1 is coupled to a selection signal line Select, and the source is coupled to the data line Vdata.
In the operation of the pixel of the above structure, when the transistor M1 is turned ON in response to the selection signal Select applied to the gate of the switching transistor M1, the data voltage Vdata is applied to the gate of the driving transistor M2 through the data line. In response to the data voltage Vdata applied to the gate, a current flows to the organic EL element (OLED) through the transistor M2 to emit light.
The current flowing to the organic EL element (OLED) is given by the following equation:
                              I          OLED                =                                            β              2                        ⁢                                          (                                  Vgs                  -                  Vth                                )                            2                                =                                    β              2                        ⁢                                          (                                  Vdd                  -                  Vdata                  -                                                          Vth                                                                      )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            where IOLED is the current flowing to the organic EL element (OLED); Vgs is the voltage between the source and gate of the transistor M2; Vth is the threshold voltage of the transistor M2; Vdata is the data voltage; and β is a constant.
As can be seen from the equation 1, according to the pixel circuit of FIG. 1, the current corresponding to the applied data voltage Vdata is supplied to the organic EL element (OLED), which emits light by the supplied current.
Typically, the pixel driving voltage Vdd is constructed as a horizontal or vertical line for supplying the power to the driving transistor of each cell. When the pixel driving voltage Vdd is constructed as a horizontal line as illustrated in FIG. 2 and there are many turned-on driving transistors in the cell coupled to each branched Vdd line, a high current flows to the corresponding Vdd line, and the voltage difference between the right and left sides of the line increases.
This voltage drop in the voltage line Vdd is proportional to the amount of current, which is dependent upon the number of turned-on pixels among the pixels coupled to the corresponding line. So, the voltage drop is also changed depending on the number of turned-on pixels. In FIG. 2, the driving voltage Vdd applied to the right-handed pixel of the line is lower than the driving voltage Vdd applied to the left-handed pixel, and the voltage Vgs applied to the driving transistor located at the right-handed pixel is lower than the voltage Vgs applied to the driving transistor at the left-handed pixel, thereby causing a difference in the amount of current flowing to the transistors and hence a brightness difference.
Despite having the same voltage Vgs, the amount of current supplied to the organic EL element (OLED) changes causing a brightness difference, due to changes in the threshold voltage Vth of the TFT. Changes in the threshold voltage Vth of the TFT occurs due to the non-uniformity of the manufacturing process.
FIG. 3 is a circuit diagram of a pixel circuit derived to solve the above problem and to avoid the non-uniformity of brightness caused by the variation of the threshold voltage Vth of the driving transistor. FIG. 4 is a driving timing diagram for the circuit of FIG. 3.
In this circuit, however, the data voltage for the driving transistor M2 must be equal to the driving voltage Vdd while AZ signal is LOW. The source-gate voltage of the driving transistor is given by the following equation:
                    Vgs        =                  Vth          +                                    C1                              C1                +                C2                                      ⁢                          (                              Vdd                +                Vdata                            )                                                          [                  Equation          ⁢                                          ⁢          2                ]            where Vth is the threshold voltage of the transistor M2; Vdata is the data voltage; and Vdd is the driving voltage.
As can be seen from the equation 2, there is a problem because the swing width of the data voltage or the value of the capacitor C1 must be large enough because the data voltage is divided by the capacitors C1 and C2.